This invention relates to a high speed gas chromatography system and particularly to such systems employing column bifurcation and tunable selectivity for improved rapid separation of components of an analyte mixture.
Gas chromatography is unsurpassed in its selectivity, sensitivity, and cost effectiveness. It is applicable for at least several hundred thousand compounds of low to moderate boiling point, including compounds in the C.sub.1 to C.sub.5 range. The process is also unique in its ability to obtain complete speciation of complex mixtures of compounds.
In gas chromatography analysis the analyte mixture is separated into its components by eluting them from a column having a sorbent by means of a moving gas. In gas-liquid chromatography, which is a type in widespread use at present, the column comprises a nonvolatile liquid or solid sorbent coated as a thin layer on an inner support structure, generally the inside surface of a capillary tube. The moving gas phase, called the carrier gas, flows through the chromatography column. The analyte partitions itself between the moving gas phase and the sorbent and moves through the column at a rate dependent upon the partition coefficients or solubilities of the analyte components. The analyte is introduced at the entrance end of the column within the moving carrier gas stream. The components making up the analyte become separated along the column and elute at intervals characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the analytical column responds to the presence of the analyte components. Upon combustion of the eluted components at the FID, charged species are formed in the flame. The flame characteristics are monitored through a biased ion detector which, along with associated signal processing equipment, produces a chromatogram which is a time versus detector signal output curve. The trace for complex mixtures includes numerous peaks of varying intensity. Since individual constituents of the analyte produce peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of a chromatogram.
While gas chromatography systems presently available perform satisfactorily, designers of such systems are continually attempting to optimize the capabilities of this separation procedure. Of particular interest is providing high speed gas chromatography which is advantageous in providing process stream control in industrial applications and in monitoring transient processes, for example, internal combustion engine exhaust gas compositions. The use of special inlet systems when combined with relatively short separation columns operated at unusually high carrier gas flow rates, has allowed separation of relatively simple mixtures on a time scale of a few seconds. However, some samples require much longer separation times because of the probability of co-eluting components. This probability is the result of the inevitable decrease in resolution when separation times are drastically reduced. To make high speed separation more practical, and to be able to apply fast gas chromatography techniques to a wider range of potential applications, it is necessary to enable adjustment of the selectivity of the system for specific sets of target compounds.
In view of the foregoing, an objective of this invention is to improve the selectivity of high speed chromatography by using high speed column bifurcation switching and pressure tuning techniques. Certain groups of components in a mixture which are not separated on an initial pre-separation column are switched to one or more columns which may have a different stationary phase where they are separated. High speed precision switching is achieved using this invention without the use of valves in the sample flow path. The pressure at the switching point is adjustable in accordance with this invention to improve the selectivity of the separation. By changing this pressure, the influence of the various columns on the separation process is changed. This can result in the shifting of overlapping peaks in such a way that they no longer overlap.
Preferably, in accordance with this invention, column switching is achieved through the use of a computer control system. Through such precision column switching and pressure tuning techniques, the selective transfer of components to an appropriate column can be provided which will optimize their separation and will thus allow a more efficient utilization of the available separating power of short columns operated at high carrier gas flow rates. The systems in accordance with this invention provide for the selectivity tuning of a tandem column combination to provide adjustments of the times at which components are eluted to ensure lack of overlapping or co-eluting components.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.